This invention relates to the manufacture of semiconductor devices and, more particularly, to irradiation of semiconductor devices to improve the electrical characteristics thereof.
Since their introduction, power semiconductors have become more widely used, as circuit designers have continued to employ them in new and different ways. As new applications for these devices are developed, greater emphasis is placed on improving the performance of the devices under demanding conditions of operation. Different applications require that the emphasis be placed on different characteristics of the device. For example, some potential applications will become commercially feasible only if the cost of the devices can be reduced. Other applications require that devices be able to withstand higher reverse voltages, or that they switch from a conductive to a nonconductive state more rapidly. Thus, the device designer is faced with a multi-faceted problem.
One of the principal concerns of the device designer is the interrelationship among the various device characteristics. For example, certain methods of controlling the device turnoff time are known. Gold or platinum can be diffused into the device body, or the device body can be irradiated. Functionally, these two methods are similar in that both create recombination centers that stimulate carrier recombination, thus depleting the semiconductor device of mobile minority carriers more rapidly. However, these methods also affect other device characteristics. For example, the gold diffusion generally increases the the device leakage current and is costly. Irradiation typically increases the semiconductor device's forward voltage drop.
Work has been done in the past to alleviate the aforementioned problems. For example, see U.S. Pat. Nos. 3,941,625 and 3,943,013, both Kennedy et al., and the pending application of Moyson, Ser. No. 616,404, filed Sept. 24, 1975, and entitled, "Gold Diffused Thyristor and Method of Manufacture," and the pending application of Kennedy, et al., Ser. No. 654,282, filed Feb. 2, 1976, and entitled, "Glass Passivated Gold Diffused Rectifier Pellet and Method for Making". These patents and applications, assigned to the assignee of the present invention, deal with methods of selectively controlling gold or platinum diffusion so that the impurity is put only where it is needed to stimulate carrier recombination, thus minimizing its affect on other device properties.
Work has also been done on alleviating the undesired adverse effects of device irradiation. For example, see U.S. Pat. Nos. 3,881,964 and 3,888,701. These two patents teach an annealing step following device irradiation. The irradiation taught by these references is carried out at room temperature. Radiation damage may cause some heating of the crystal, but it is doubtful that the crystal ever exceeds 70.degree. or 80.degree. C in temperature. In general, it is explained in the last referenced patent that as a semiconductor device is irradiated, the turnoff time exhibited by the device is decreased; however, the forward voltage drop increases. That, of course, was known in the prior art. It is further taught in the patents that annealing the devices for up to 120 hours following irradiation will cause a decrease in the forward voltage drop. There will be some increase of the recovery time due to the anneal, but not up to its original high value.
A problem which occurs when practicing the irradiate and anneal sequence described in the aforementioned patents is that an excessive amount of energy and time, and thus money, are consumed in the device manufacture. The time comsumption is, of course, obvious. Following irradiation, the devices are annealed at an elevated temperature for a time which may exceed 100 hours, and the irradiate-anneal cycle may be carried out more than once. With respect to energy consumption, heating devices, such as ovens, are high energy consumers and thus are expensive to operate for such long time periods.
It is an object of this invention, therefore, to provide a method of manufacturing power semiconductors exhibiting the rapid turnoff properties of gold doped or irradiated devices, also, to alleviate the aforementioned deficiencies of excessive forward voltage drop, high leakage current, and high cost.